So far, the handbook has covered types which are atomic objects.\nHowever, as you model more types you find yourself looking for tools which let you compose or combine existing types instead of creating them from scratch.
\nIntersection and Union types are one of the ways in which you can compose types.
\nOccasionally, you’ll run into a library that expects a parameter to be either a number or a string.\nFor instance, take the following function:
\nts/**\n * Takes a string and adds \"padding\" to the left.\n * If 'padding' is a string, then 'padding' is appended to the left side.\n * If 'padding' is a number, then that number of spaces is added to the left side.\n */\nfunctionTrypadLeft (value : string,padding : any) {\n if (typeofpadding === \"number\") {\n returnArray (padding + 1).join (\" \") +value ;\n }\n if (typeofpadding === \"string\") {\n returnpadding +value ;\n }\n throw newError (`Expected string or number, got '${padding }'.`);\n}\n\npadLeft (\"Hello world\", 4); // returns \" Hello world\"
The problem with padLeft in the above example is that its padding parameter is typed as any.\nThat means that we can call it with an argument that’s neither a number nor a string, but TypeScript will be okay with it.
\nts// passes at compile time, fails at runtime.\nletTryindentedString =padLeft (\"Hello world\", true);
In traditional object-oriented code, we might abstract over the two types by creating a hierarchy of types.\nWhile this is much more explicit, it’s also a little bit overkill.\nOne of the nice things about the original version of padLeft was that we were able to just pass in primitives.\nThat meant that usage was simple and concise.\nThis new approach also wouldn’t help if we were just trying to use a function that already exists elsewhere.
Instead of any, we can use a union type for the padding parameter:
\nts/**\n * Takes a string and adds \"padding\" to the left.\n * If 'padding' is a string, then 'padding' is appended to the left side.\n * If 'padding' is a number, then that number of spaces is added to the left side.\n */\nfunctionTrypadLeft (value : string,padding : string | number) {\n // ...\n}\n\nletindentedString =padLeft (\"Hello world\",true );\nArgument of type 'boolean' is not assignable to parameter of type 'string | number'.2345Argument of type 'boolean' is not assignable to parameter of type 'string | number'.
A union type describes a value that can be one of several types.\nWe use the vertical bar (|) to separate each type, so number | string | boolean is the type of a value that can be a number, a string, or a boolean.
If we have a value that is a union type, we can only access members that are common to all types in the union.
\n\ntsinterfaceTryBird {\nfly (): void;\nlayEggs (): void;\n}\n\ninterfaceFish {\nswim (): void;\nlayEggs (): void;\n}\n\ndeclare functiongetSmallPet ():Fish |Bird ;\n\nletpet =getSmallPet ();\npet .layEggs ();\n\n// Only available in one of the two possible types\npet . \nProperty 'swim' does not exist on type 'Bird | Fish'.\n Property 'swim' does not exist on type 'Bird'.2339Property 'swim' does not exist on type 'Bird | Fish'.\n Property 'swim' does not exist on type 'Bird'.swim ();
Union types can be a bit tricky here, but it just takes a bit of intuition to get used to.\nIf a value has the type A | B, we only know for certain that it has members that both A and B have.\nIn this example, Bird has a member named fly.\nWe can’t be sure whether a variable typed as Bird | Fish has a fly method.\nIf the variable is really a Fish at runtime, then calling pet.fly() will fail.
A common technique for working with unions is to have a single field which uses literal types which you can use to let TypeScript narrow down the possible current type. For example, we’re going to create a union of three types which have a single shared field.
\n\n\ntstype NetworkLoadingState = {\n state: \"loading\";\n};\n\ntype NetworkFailedState = {\n state: \"failed\";\n code: number;\n};\n\ntype NetworkSuccessState = {\n state: \"success\";\n response: {\n title: string;\n duration: number;\n summary: string;\n };\n};\n\n// Create a type which represents only one of the above types\n// but you aren't sure which it is yet.\ntype NetworkState =\n | NetworkLoadingState\n | NetworkFailedState\n | NetworkSuccessState;
All of the above types have a field named state, and then they also have their own fields:
NetworkLoadingState | \n NetworkFailedState | \n NetworkSuccessState | \n
|---|---|---|
| state | \nstate | \nstate | \n
| \n | code | \nresponse | \n
Given the state field is common in every type inside NetworkState - it is safe for your code to access without an existence check.
With state as a literal type, you can compare the value of state to the equivalent string and TypeScript will know which type is currently being used.
NetworkLoadingState | \n NetworkFailedState | \n NetworkSuccessState | \n
|---|---|---|
\"loading\" | \n \"failed\" | \n \"success\" | \n
In this case, you can use a switch statement to narrow down which type is represented at runtime:
\ntstypeTryNetworkState =\n |NetworkLoadingState \n |NetworkFailedState \n |NetworkSuccessState ;\n\nfunctionnetworkStatus (state :NetworkState ): string {\n // Right now TypeScript does not know which of the three\n // potential types state could be.\n\n // Trying to access a property which isn't shared\n // across all types will raise an error\nstate . \nProperty 'code' does not exist on type 'NetworkState'.\n Property 'code' does not exist on type 'NetworkLoadingState'.2339Property 'code' does not exist on type 'NetworkState'.\n Property 'code' does not exist on type 'NetworkLoadingState'.\n // By switching on state, TypeScript can narrow the union\n // down in code flow analysis\n switch (code ;state .state ) {\n case \"loading\":\n return \"Downloading...\";\n case \"failed\":\n // The type must be NetworkFailedState here,\n // so accessing the `code` field is safe\n return `Error ${state .code } downloading`;\n case \"success\":\n return `Downloaded ${state .response .title } - ${state .response .summary }`;\n }\n}
We would like the compiler to tell us when we don’t cover all variants of the discriminated union.\nFor example, if we add NetworkFromCachedState to NetworkState, we need to update logger as well:
\ntstypeTryNetworkFromCachedState = {\nstate : \"from_cache\";\nid : string\nresponse :NetworkSuccessState [\"response\"]\n}\n\ntypeNetworkState =\n |NetworkLoadingState \n |NetworkFailedState \n |NetworkSuccessState \n |NetworkFromCachedState ;\n\nfunctionlogger (s :NetworkState ) {\n switch (s .state ) {\n case \"loading\":\n return \"loading request\";\n case \"failed\":\n return `failed with code ${s .code }`;\n case \"success\":\n return \"got response\"\n }\n}
There are two ways to do this.\nThe first is to turn on --strictNullChecks and specify a return type:
\ntsfunctionTrylogger (s : \nFunction lacks ending return statement and return type does not include 'undefined'.2366Function lacks ending return statement and return type does not include 'undefined'. switch (NetworkState ): string {s .state ) {\n case \"loading\":\n return \"loading request\";\n case \"failed\":\n return `failed with code ${s .code }`;\n case \"success\":\n return \"got response\"\n }\n}
Because the switch is no longer exhaustive, TypeScript is aware that the function could sometimes return undefined.\nIf you have an explicit return type string, then you will get an error that the return type is actually string | undefined.\nHowever, this method is quite subtle and, besides, --strictNullChecks does not always work with old code.
The second method uses the never type that the compiler uses to check for exhaustiveness:
\ntsfunctionTryassertNever (x : never): never {\n throw newError (\"Unexpected object: \" +x );\n}\n\nfunctionlogger (s :NetworkState ): string {\n switch (s .state ) {\n case \"loading\":\n return \"loading request\";\n case \"failed\":\n return `failed with code ${s .code }`;\n case \"success\":\n return \"got response\";\n default: \n returnassertNever ()\nArgument of type 'NetworkFromCachedState' is not assignable to parameter of type 'never'.2345Argument of type 'NetworkFromCachedState' is not assignable to parameter of type 'never'. }\n} s
Here, assertNever checks that s is of type never — the type that’s left after all other cases have been removed.\nIf you forget a case, then s will have a real type and you will get a type error.\nThis method requires you to define an extra function, but it’s much more obvious when you forget it because the error message includes the missing type name.
Intersection types are closely related to union types, but they are used very differently.\nAn intersection type combines multiple types into one.\nThis allows you to add together existing types to get a single type that has all the features you need.\nFor example, Person & Serializable & Loggable is a type which is all of Person and Serializable and Loggable.\nThat means an object of this type will have all members of all three types.
For example, if you had networking requests with consistent error handling then you could separate out the error handling into it’s own type which is merged with types which correspond to a single response type.
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